This invention relates in general to gas sensors and in particular to a new and useful gas sensor having a solid electrolyte of tetragonal zirconium dioxide.
The invention particularly concerns a sensor for measuring partial pressures of gases, especially partial pressures of oxygen, with an ion-conducting solid electrolyte of zirconium dioxide as the sensor element, which is arranged between two electrodes for measuring an electrically measurable quantity.
A similar sensor arrangement of this kind has been known from German OS No. 28 37 593. In the previously known gas sensors using zirconium dioxide as the solid electrolyte, use is principally made of the conductivity of the cubic modification of zirconium dioxide. Additions of calcium, magnesium or yttrium oxide in high concentrations of about 8-9 mol % are used to stabilize the cubic zirconium dioxide. Such a stabilized cubic modification of zirconium dioxide is used for reasons of thermoshock stability and mechanical stability of the gas sensor. Because of the lower valency of the dopant metal ions, vacancies are generated in the oxygen sublattice of the solid electrolyte, across which a transfer of gas ions, especially oxygen ions, is possible by means of a vacancy mechanism. The known gas sensors are used to measure the oxygen in the exhaust gases from internal-combustion engines or in flue-gas from furnaces. The optimal operating temperature for the known gas sensors lies between 800.degree. and 1000.degree.. In operation, these sensors have to withstand high temperature changes, so that a high demand is put on the thermoshock stability of the solid electrolyte of zirconium dioxide used.
Since the specific resistivity of stabilized cubic zirconium dioxide increases rapidly with decreasing temperature, because of the high activation energy of the conductivity, it was attempted to lower the cell impedance by reducing the electrolyte thickness, using methods from thin- or thick-film technology. However, the operating temperature of the sensors could not thereby be satisfactorily decreased. The reason for this clearly lies in that the rate determining step of the incorporation of gas molecules at the electrodes from the gas phase in the solid electrolyte cannot be modified by this approach. The transfer of the gas into the lattice structure of the solid electrolyte is severely inhibited at temperature significantly below 500.degree. C.
It is known to add a small proportion of tetragonal zirconium dioxide to the stabilized cubic zirconium dioxide for reasons of the increase in mechanical stability, thereby creating a heterogenious two-phase system. However, the ion conductivity is in this case likewise achieved exclusively through the stabilized cubic modification of zirconium dioxide.